After four years in office, Environmental Protection Agency Administrator Lisa Jackson said on Thursday that she will step down after President Obama’s State of the Union address in January, inviting speculation about who will be named as her successor.

Jackson’s four-year tenure at the agency was busy and productive, but it also was controversial. Since Jackson was confirmed to head EPA in 2009, the agency has undertaken a number of significant and divisive measures, including setting new standards to clean up mercury and other toxic emissions from coal-fired power plants, and setting new standards to limit fine particle soot in the air. EPA played a lead role in establishing new fuel-economy and greenhouse-gas standards for motor vehicles. In 2009, the agency reversed findings made under the Bush administration, declaring that climate change poses a real threat to public health and the environment.

This active administrative role made her a target for lawmakers and pundits who see EPA regulations as symptomatic of the Obama administration’s government overreach, but her leadership did score points within the environmental community, which on Thursday lamented her imminent departure.

“There has been no fiercer champion of our health and our environment than Lisa Jackson, and every American is better off today than when she took office nearly four years ago,” said Frances Beinecke, president of the Natural Resources Defense Council. “Lisa leaves giant shoes to fill.”

As her exit didn’t come as too much of a surprise to the environmental community, they have long been speculating about whom Obama will choose as her successor.

Following are the candidates most frequently mentioned as the next potential EPA administrator:

Bob Perciasepe

Perciasepe, the agency’s deputy administrator and chief operating officer, is widely considered to be the front-runner for Jackson’s job. Perciasepe has had a wide-ranging career and is considered one of the most qualified for the post. In the Clinton administration, he was first appointed to serve as the nation’s top water official and later as the senior official responsible for air quality. Before his appointment to EPA in 2009, he was chief operating officer at the National Audubon Society and has served as secretary of the Environment for the state of Maryland.

“I think Bob Perciasepe is the leading, and probably most deserving, candidate. He has worked at every level of government, and in almost every major part of EPA—air, water, and deputy administrator. In each job, he has performed with distinction,” William Becker, executive director of the National Association of Clean Air Agencies, told National Journal on Thursday. "The others are also very worthy candidates, but my money is on Bob!" Perciasepe is not only considered a front-runner due to his career breadth but also because he is respected in both environmental and industry circles, making for a potentially smoother confirmation process. “Both environmental groups and industry see him as a straight shooter,” said Clean Air Watch President Frank O'Donnell.

Mary Nichols

Nichols, chairwoman of the California Air Resources Board, would be an ambitious and possibly controversial choice to head EPA, but she has been right at the center of successor speculation. In her current post since 2007, Nichols has led the charge on the Golden State’s cap-and-trade program for greenhouse-gas emissions that has come under intense attack from major oil companies. Her work on cap-and–trade has brought her into close contact with Obama administration officials who worked on a comprehensive climate-change bill in the last term, and she is known to have close ties with Senate Environment and Public Works CommitteeChairwoman Barbara Boxer, D-Calif., who in 2008 supported a Nichols nomination.

Nichols previously served as assistant administrator for EPA’ s air and radiation program in the Clinton administration and as secretary for California's Resources Agency from 1999 to 2003. Ahead of that, Nichols served as director of Institute of the Environment at the University of California (Los Angeles).

Gina McCarthy

EPA's air chief has also been named as one of Jackson’s potential successors. As assistant administrator for the Office of Air and Radiation at the EPA, McCarthy has been behind some of the Obama administration’s toughest clean-air policies and has often testified in lieu of Jackson at congressional committee hearings on the agency’s controversial regulations.

Before her confirmation, McCarthy served as commissioner of the Connecticut Department of Environmental Protection and had extensive involvement with the Regional Greenhouse Gas Initiative, a multistate pact to cap and cut emissions. McCarthy, who is originally from Boston, served as an environmental regulator under Mitt Romney and the four previous Massachusetts governors.

Bradley Campbell

Campbell, who was commissioner of New Jersey’s Environmental Protection Department from 2002 to 2006, was Jackson’s boss before she came to Washington. Campbell currently heads a private law and consulting practice focused on environment, energy, and entrepreneurship in New York, but his résumé does include extensive experience in Washington.

Before his work in the New Jersey department, Campbell served as EPA regional administrator for the Mid-Atlantic region, enforcing federal environmental laws in five states and the District of Columbia. He also spent five years in the Clinton administration as associate director of the White House’s Council on Environmental Quality.

Kathleen McGinty

McGinty, former head of Pennsylvania’s Department of Environmental Protection, is considered to be another potential nominee for Jackson’s post. McGinty began as a protégé of former Vice President and current environmental activist Al Gore, serving as his aide when he was in the Senate. She accompanied Gore to the White House and became the first woman to chair the White House Council on Environmental Quality and founded and headed the White House Office on Environmental Policy.

McGinty’s experience in climate protection has earned her respect among many in the environmental community, but she has also been praised for her record in working with industry and utilities in Pennsylvania. She is the founding partner at Peregrine Technology Partners, a firm focused on the commercialization of clean technologies. McGinty is also a director at wholesale power company NRG Energy.

Heather Zichal

Zichal, the White House’s top aide on energy and climate issues, is a popular choice among some environmental groups. She has been in the Obama administration since 2009, serving as deputy to Obama’s top climate and energy policy aide, Carol Browner. Zichal took over upon Browner’s departure in early 2011, when officials said they would bring the energy office and the Office of Health Reform under the umbrella of the Domestic Policy Council. Zichal spent more than eight years working on Capitol Hill in three different congressional offices, including as legislative director for Sen. John Kerry, D-Mass. She also served as a top energy and environment adviser to the 2004 Kerry presidential campaign and the Obama 2008 presidential campaign.

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